1. Field of the Invention
This invention relates generally to above atmospheric pressure gas discharge lasers, and more specifically to pulsed transverse electric discharge excimer or rare gas halide lasers.
2. Discussion of the Prior Art
A rare gas halide laser includes a multi-atmospheric pressure mixture predominantly consisting of a rare gas buffer such as helium, neon or argon (more than 80%), a reactive rare gas such as Krypton or Xenon (usually less than several percent), and a trace amount of halogen gas such as HCl or F.sub.2 (usually less than 1%). The halogen component of rare gas halide lasers is very reactive. To the extent that this component reacts with impurities and other materials in the laser housing, the gas mixture degrades resulting in a substantial reduction in the pulse laser energy produced by gas discharge. If the chamber cannot be thoroughly cleaned of impurities, this degradation greatly reduces the life of the gas mixture so that an entire gas exchange must be conducted. With excimer lasers of the past, this gas exchange has typically been required on a weekly or monthly basis depending on usage and the materials of construction. So frequent has been this requirement for gas exchange that customers have been required to purchase elaborate equipment and to train their own personnel in the exchange process.
The laser chamber is formed by a vacuum/high pressure containment vessel or housing for the gas mixture, with windows at each end of the vessel to allow for optical feedback and coupling of laser light. Inside the laser housing two transverse longitudinal discharge electrodes extend along the length of the chamber. These electrodes produce a pulsed electric discharge transverse to the electrodes which excites the gas mixture and produces the excited rare gas halide molecule which supports stimulated laser emission. In order to produce a uniform efficient discharge at pulse repetition frequencies, the laser gas is circulated across the discharge region by a fan assembly. The initial main discharge pulse is facilitated by a preionization assembly which produces a uniform density of electrons within the discharge region. Both the main discharge and the preionization apparatus require electrical connections having a low inductance in order to produce uniform stable discharge excitations. A heat exchanger may also be provided in the laser chamber to remove excess heat resulting from higher duty cycles.
It has been appreciated that the laser head must be formed from a material which is compatible (non-reactive) with the halogen in the gas mixture. Nickel has been a preferred material although it is generally felt to be too expensive to form the entire structure of this material. As a result, a base material such as aluminum has been plated with nickel in order to present a nonreactive surface to the gas mixture. Unfortunately, even with tight control of the process, the plating has tended to blister and otherwise degrade, in which case the underlying aluminum has undesirably reacted with the halogen gas.
In order to meet both vacuum and high pressure criteria, the containment vessel has been formed as a cylinder with the discharge electrodes extending longitudinally between the ends of the cylinder. The cylinder has been sealed at its opposing ends by a pair of end plates. In prior tubular laser designs where it has been necessary to introduce high voltage along the length of the electrodes, large lateral sections of the cylinder have been removed and replaced with flat plates to support the associated electrical components. This configuration has significantly degraded the structural integrity of the cylindrical vessel, often below ASME standards for a pressure vessel. The welding required for this design has required a pressure code certified welder who may be knowledgeable about pressure safety but is not necessarily aware of clean, high vacuum considerations.
In order to maintain the structure of the cylinder, prior lasers have supported the interior laser components, such as the electrodes, on one of the end plates. These designs have relied on a single electrical discharge connector, passing through the same end plate to energize the electrodes. Since electric discharge specifications require balanced inductance over the entire length of the transverse discharge electrode, this design has required that capacitors be mounted within the laser to balance the discharge and lower the discharge inductance. These capacitors tend to degrade when directly exposed to the halogen gas, and unfortunately servicing requires disassembly of the entire laser head. Although attempts have been made to coat the capacitors with halogen compatible materials, such efforts have fallen short in the absence of suitable coatings.
The mounting of components on an end plate has also been attempted by providing a sealed tube within the cylindrical vessel. This interior tube has been isolated from the gas mixture so that the electrical components within the tube do not react to reduce gas lifetime. This design has added extra expense and size to the laser head, and has been very difficult to service since the laser chamber must be opened to access the electrical components. Not only is this servicing procedure labor intensive, but care must be taken not to allow the interior parts to be exposed to air. Such exposure could cause material corrosion and require extensive laser chamber cleaning and reconditioning after reassembly.
In the past, metal seals have been avoided at the end flanges because they tend to chip the nickel plating. In most cases O-ring seals have been used, but these tend to outgas halocarbons which opacify the windows and also reduce gas lifetimes. In 1986, one of the present inventors unsuccessfully used high vacuum knife edge flanges for a research medical laser. In this design, the standard ASME pressure welding techniques required both inside and outside continuous partial penetration welds for the cylinder and flange. This led to contamination from a leak in the inside weld and which was impossible to locate using a leak checker.
Bearings associated with the fan assembly have also been critical components. Generally non-lubricated bearings are preferred but these also must be formed from materials which are halogen compatible. Ceramic bearings have been used but with unsatisfactory results, leading to premature seizure.
A commercial bearing from Barden Corp. using a non-metallic bearing race of BarTemp material serves as a reliable dry bearing for halogen compatibility. Also in prior designs ground return plates have been provided in the discharge region associated with the electrodes of the laser. These plates have been allowed to contact the laser vessel over wide areas of the container. As a consequence, the vessel has tended to radiate electromagnetic interference which in turn has upset digital control electronics in proximity to the laser.
Notwithstanding the appreciation of the prior art that a laser head must be formed out of compatible materials, and must meet both high vacuum and pressure vessel requirements in a commercial environment, improvements have been necessary to achieve a structure which will meet the requirements of electrical design manufacturability, serviceability, reliability and safety for medical applications.